1. Field of the Invention
The present invention relates to a vibration isolator and a vibration isolator in combination with an exposure apparatus. More specifically the present invention relates to a so-called "active type" vibration isolator in which a vibration isolating stand is driven by actuators so that the vibration of the vibration isolating stand is canceled, and also an exposure apparatus that is equipped with the vibration isolator.
2. Discussion of the Related Art
As precision requirements have increased in high-precision instruments, such as step-and-repeat-type reducing projection exposure devices (so-called "steppers"), it has become necessary to isolate micro-vibrations acting on the base plate (vibration isolating stand) of such devices from the installation floor at the micro-G level. Various types of dampers, such as mechanical dampers, in which compression coil springs are placed in a damping liquid and pneumatic dampers, have been used as vibration isolating pads that support the vibration isolating stand. The vibration isolating pads also are capable of performing some centering functions, especially in air spring vibration isolators equipped with pneumatic dampers, wherein the spring constant can be set at a small value so that vibrations exceeding approximately 10 Hz or higher can be isolated. Accordingly, such vibration isolators are widely used for the support of precision instruments. Recently, active vibration isolators have been proposed in order to break through the limits inherent in passive vibration isolators. The active vibration isolators are vibration isolators in which the vibration of the vibration isolating stand is detected by sensors and vibration control is accomplished by driving actuators on the basis of the output of the sensors. Such vibration isolators are capable of producing an ideal vibration isolating effect with no resonance peak in the low-frequency control region.
In steppers, an XY-stage (wafer stage), which undergoes a large acceleration and deceleration, is mounted on a base plate held by vibration isolating pads. The center of gravity of the main body of the exposure apparatus moves simultaneously with the movement of the XY-stage. In an active type vibration isolator, when the position of the center of gravity of the main body changes as a result of the movement of the wafer stage, the initial positioning is performed by means of a position control loop. If the amount of movement of the stage is large, the corresponding change in the position of the center of gravity of the main body also is large, so that the main body experiences tilting. The amount of this inclination increases with an increase in the amount of change in the position of the center of gravity of the main body, so that the driving force required in the actuators in order to correct this is also increased. In active vibration isolators used in conventional exposure apparatus, the output values of the respective displacement sensors obtained when the respective stages, i.e., wafer X-stage, wafer Y-stage, and in scan-type exposure apparatus, the reticle stage; used for the target setting values, are used in order to correct the inclination of the base plate (vibration isolating stand) and limit vibration. Furthermore, the target setting ranges are uniformly set on the basis of permissible values with six degrees of freedom at the emission position of the light exposure source and the receiving positions of the reticle and wafer loader.
The amount of heat generated by the actuators in the vibration isolator is especially large when the main body is tilted, so that the environment in which the exposure apparatus is placed undergoes a large temperature variation. This temperature variation in the environment has an effect on the measurement precision of the laser interferometers that measure the position of the XY-stage. The temperature variation therefore leads to a deterioration in the stage positioning precision.